getGovernorTokens returns null when there is no governor

[danieldk]

DependencyParsingLayerStored#getGovernorTokens returns null when a token has no governors. The proper thing to do would be to return an empty array (with a guarantee that the method does not return null).

Even if this cannot be changed anymore (I am not sure how much code depends on this behaviour), it a hint in the Javadoc would be nice.